Because modern integrated circuits as so small, even the slightest damage or imperfection can potentially destroy the device. Therefore, integrated circuits are rigorously inspected for defects, so as to quickly find and eliminate the sources of such before a great amount of damage is done.
As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
Conventionally, defects in integrated circuits are detected during a substrate inspection by forming optical or other types of images of two adjacent, identical integrated circuits, arrays, or fields of view, and then subtracting the image of one die or cell from the image of the other die or cell. After the subtraction, at least one threshold is used to determine if there is a significant mismatch between the corresponding pixels of the two images, as determined by the subtraction image, which mismatch is indicative of a defect on at least one of the dice.
As the integrated circuit substrate processing gets more and more complicated, the conventional defect detection methods become less and less effective, because of the process variation and substrate noise that exhibit severe color variation and grains on the images collected from the devices on the substrate.
What is needed, therefore, is a system which overcomes, at least in part, at least some of the problems described above.